1. Field of the Invention
The present invention relates to an improvement of a variable compression ratio device of an internal combustion engine, wherein a piston includes a piston inner part which is connected to a connecting rod via a piston pin and a piston outer part which is fitted on an outer periphery of the piston inner part so as to be only slidable in an axial direction. The piston outer part is movable between a low compression ratio position near the piston inner part and a high compression ratio position near a combustion chamber, with an outer end surface of the piston outer part facing the combustion chamber. A cam mechanism is provided between the piston inner part and the piston outer part that alternately gives the low compression ratio position and the high compression ratio position to the piston outer part. An actuator is provided in the piston inner part to operate the cam mechanism. The cam mechanism includes a lift member which is supported by the piston inner part and is moved by the actuator alternately to a lift release position and a lift position. A plurality of first cam top portions are projectingly provided integrally on a top surface of the lift member. A plurality of second cam top portions are projectingly provided on an undersurface of a head part of the piston outer part. In the lift release position of the lift member, the first cam top portions and the second cam top portions are meshed with each other to move the piston outer part to the low compression ratio position. In the lift position of the lift member, the first cam top portions and the second cam top portions cause their top surfaces to abut on each other to move the piston outer part to the high compression ratio position.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2004-44512 discloses a variable compression ratio device for an internal combustion engine.
In the conventional variable compression ratio device of an internal combustion engine, a cam mechanism moves a piston outer part alternately to a low compression ratio position and a high compression ratio position. The cam mechanism includes a lift member which is supported by a piston inner part to move between a lift release position and a lift position. A plurality of first cam top portions are projectingly provided integrally on a top surface of the lift member. A plurality of second cam top portions are projectingly provided on an undersurface of a head part of the piston outer part. In the lift release position of the lift member, the first cam top portions and the second cam top portions are meshed with each other to move the piston outer part to a low compression ratio position. In the lift position of the lift member, the first cam top portions and the second cam top portions cause their top surfaces to abut on one another to move the piston outer part to a high compression ratio position. Therefore, in either of the low compression ratio position and the high compression ratio position of the piston outer part, the piston inner part and the piston outer part abut on each other in a large area via the cam mechanism, so that heat transfer from the piston outer part at a high temperature to the piston inner part at a low temperature is excellently performed. Thus, the variable compression ratio device has an advantage of being capable of enhancing a cooling performance of the piston.
However, since a skirt part which is slidably fitted to an inner peripheral surface of a cylinder bore to stabilize the posture of the piston is formed at the piston outer part, when the piston outer part switches the position between the low compression ratio position and the high compression ratio position by using its inertia force, the piston outer part is slid accompanying the skirt part on the inner peripheral surface of the cylinder bore, so that a delay in switching of the position sometimes occurs due to sliding resistance of the skirt part. In addition, since the piston inner part is slidably fitted to the inner peripheral surface of the piston outer part over its entire outer peripheral surface, the fitting area of the piston inner part and the piston outer part is large, thereby increasing the weight of the piston.